Contributions of charged residues in a cytoplasmic linking region to Na channel gating.

Na channels inactivate quickly after opening, and the very highly positively charged cytoplasmic linking region between homologous domains III and IV of the channel molecule acts as the inactivation gate. To test the hypothesis that the charged residues in the domain III to domain IV linker have a role in channel function, we measured currents through wild-type and two mutant skeletal muscle Na channels expressed in Xenopus oocytes, each lacking two or three charged residues in the inactivation gate. Microscopic current measures showed that removing charges hastened activation and inactivation. Macroscopic current measures showed that removing charges altered the voltage dependence of inactivation, suggesting less coupling of the inactivation and activation processes. Reduced intracellular ionic strength shifted the midpoint of equilibrium activation gating to a greater extent, and shifted the midpoint of equilibrium inactivation gating to a lesser extent in the mutant channels. The results allow the possibility that an electrostatic mechanism contributes to the role of charged residues in Na channel inactivation gating.